Tracking system and method

ABSTRACT

Tracking system for guiding an optical beam on tracks on an optical disc, said tracking system comprising a photodetector (A1, A2) for detecting optical beams derived from said optical beam, said photodetector generating a first output signal (A) and a second output signal (B), said tracking system comprising first processing means for generating a first differential signal (PP(DC)) corresponding to the low-frequency part of a difference between said first and second output signals. The tracking system comprises second processing means for generating a tracking error signal (PP(AC/DC)) defined by the addition of said first differential signal (PP(DC)) to a second differential signal (PP′(AC)), said second differential signal corresponding to a fraction of the difference in amplitude of the high-frequency components of said first and second output signals. Use: Optical disc player/writer.

FIELD OF THE INVENTION

The invention relates to a tracking system for guiding an optical beamon tracks on an optical disc, said tracking system comprising aphotodetector for detecting optical beams derived from said opticalbeam, said photodetector generating a first output signal and a secondoutput signal, said tracking system comprising first processing meansfor generating a first differential signal corresponding to thelow-frequency part of a difference between said first and second outputsignals.

The invention may be used in the field of optical recording.

BACKGROUND OF THE INVENTION

A method of maintaining a spot on tracks of an optical disc is known asDC push-pull method. This method involves the generation of a trackingerror signal referred to as push-pull signal. Said tracking error signalcorresponds to the default error signal in the radial axis of the discand is caused by the interaction of the spot with the groove or someother tracking structure placed on the disc surface. A tracking servoadjusts the radial position of the spot to keep the push-pull signal ata predetermined value. Generally, the spot is on the track for thezero-crossing points of the push-pull signal.

FIG. 1 depicts the implementation of the known DC push-pull method. Itincludes a photodetector comprising two areas A1 and A2 for detectingreflected or transmitted beams of the optical spot. This photodetectorgenerates two output signals A and B that are filtered by low-passfilters LPF1 and LPF2, resulting in low-frequency signals A(DC) andB(DC). Subtracting means SUB1 determine the difference of the signalsA(DC) and B(DC) for the purpose of generating the low-frequencypush-pull signal PP(DC) that is used as a tracking error signal.

This prior art method is subject to limitations.

Due to the change in average reflectivity of the tracks, the amplitudeof the push-pull signal on written tracks differs from the amplitude onunwritten tracks (i.e. empty tracks). As a consequence, the slope of thepush-pull signal at the zero-crossing points varies. Moreover, offsetsin the zero-crossings area may also occur at the transition betweenwritten and unwritten tracks. Thus, this method allows the generation ofa distorted push-pull signal which introduces instabilities in thecontrol loop of the radial tracking.

Since the zero-crossing points are not precisely identified in the DCpush-pull signal, some zero-crossings may also not be taken into accountin a displacement from a first radial position to a second radialposition. In other words, this method leads to missing tracks in radialdisplacements.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to propose a tracking system in whichthe radial tracking on an optical disc is improved.

To this end, the method according to the invention is characterized inthat the tracking system comprises second processing means forgenerating a tracking error signal defined by the addition of said firstdifferential signal to a second differential signal, said seconddifferential signal corresponding to a fraction of the difference inamplitude of the high-frequency components of said first and secondoutput signals.

The invention consists in adding a fraction of a high-frequencypush-pull signal PP(AC) to the push-pull signal PP(DC) for generating atracking error signal used for radial tracking.

The variations in amplitude of the push-pull signal PP(DC) arecompensated when the optical beam is displaced between written andunwritten areas of the disc. The shape of the tracking error signal isthen close to a sine wave whatever the displacement from a written to anunwritten area, or from an unwritten to a written area. As aconsequence, the zero-crossings of this tracking error signal, used fordetecting the center of the tracks (or the center between two tracks),are precisely identified because they have substantially all the sameoffset and slope.

Because of the improvement of the radial tracking in using theinvention, the performance of optical reading and writing processes isimproved even with optical discs deviating from the norm, for examplewhen the chemical composition of the recordable or rewritable layer ofthe disc is varying. In particular, the number of missed tracks isreduced.

This tracking system is implemented by cost-effective processing meanseither in digital or analog technology, which facilitates itsintegration in consumer products such as optical disc players.

This method can be used either with the single-spot push-pull method orwith the three-spot push-pull method, which means that it can beimplemented in most existing optical disc players.

In a preferred embodiment, the tracking system according to theinvention is characterized in that it comprises adjusting means foradjusting the value of said fraction to a value which minimizes a meritfunction, said merit function being a function of a first parameter anda second parameter, said first parameter corresponding to the ratiobetween the amplitude of said first differential signal in an area withonly unwritten tracks and the amplitude of said first differentialsignal in an area with only written tracks, said second parametercorresponding to the difference in amplitude of said first differentialsignal between two adjacent tracks of which one is written and the otheris unwritten.

This characteristic allows to adapt and optimize the performance of thetracking system for the disc type and to the disc characteristics.

It is also an object of the invention to propose a method of tracking inwhich the radial tracking on an optical disc is improved.

This method of tracking comprises a photodetection step for detectingreflected or transmitted optical beams derived from said optical beam,said photodetection step generating a first output signal and a secondoutput signal, said method comprising a first processing step forgenerating a first differential signal corresponding to thelow-frequency part of a difference between said first and second outputsignals.

The method of tracking according to the invention is characterized inthat it comprises a second processing step for generating a trackingerror signal defined by the addition of said first differential signalto a second differential signal, said second differential signalcorresponding to a fraction of the difference in amplitude of thehigh-frequency components of said first and second output signals.

In a preferred mode, the method of tracking according to the inventionis characterized in that it comprises an adjusting step for adjustingthe value of said fraction to a value which minimizes a merit function,said merit function being a function of a first parameter and a secondparameter, said first parameter corresponding to the ratio between theamplitude of said first differential signal in an area with onlyunwritten tracks and the amplitude of said first differential signal inan area with only written tracks, said second parameter corresponding tothe difference in amplitude of said first differential signal betweentwo adjacent tracks of which one is written and the other is unwritten.

The invention also relates to an apparatus for reading and/or writing anoptical disc, said apparatus comprising a tracking system as describedabove.

Detailed explanations and other aspects of the invention will be givenbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

The particular aspects of the invention will now be explained withreference to the embodiments described hereinafter and considered inconnection with the accompanying drawings, in which identical parts orsub-steps are designated in the same manner:

FIG. 1 depicts a known tracking system for generating a radial trackingerror signal,

FIG. 2 depicts a tracking system according to the invention forgenerating a radial tracking error signal,

FIG. 3A shows a radial tracking error signal PP(AC/DC) generated by atracking system according to the invention, and a radial tracking errorsignal PP(DC) generated by a tracking system according to the prior artmethod,

FIG. 3B shows the track structure and properties from which the radialtracking error signals shown in FIG. 3A have been generated, and

FIG. 4 depicts an arrangement for performing an amplitude detectionaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 depicts a tracking system according to the invention forgenerating a radial tracking error signal.

The tracking system includes a photodetector comprising two areas A1 andA2 for detecting reflected beams of the optical spot. This photodetectorgenerates two output signals A and B.

For generating an improved tracking error signal PP(AC/DC), theinvention uses the high-frequency content of the output signals A and B.High-frequency components are caused by the presence of short marks onthe optical disc, said marks having different reflectivity factorsdepending on the recorded digital data. The high-frequency content ispresent for written tracks/areas and null for unwritten tracks/areas.

For written tracks, the amplitude signals A(AC) and B(AC) of thehigh-frequency signals of the two detector halves vary sinusoidally withtrack position, as does the low-frequency part used for the DC push-pullsignal PP(DC). These sinusoidal variations of the two halves are not inphase, meaning that the difference between the amplitudes of signalsA(AC) and B(AC), said differential signal being referred to as ACpush-pull signal PP(AC), also varies sinusoidally with track position.For symmetry reasons, this differential signal is zero when the spot ison the tracks (or halfway between the tracks).

The tracking system comprises first processing means for generating afirst differential signal PP(DC) corresponding to the low-frequency partof a difference between said first output signal A and said secondoutput signal B. Said first processing means comprise:

-   -   a first low-pass filter LPF1 for generating a low-frequency        signal A(DC) from signal A, the cut-off frequency of the        low-pass filter LPF1 being typically in the range of 10-100 kHz,    -   a second low-pass filter LPF2 for generating a low-frequency        signal B(DC) from signal B, the cut-off frequency of the        low-pass filters LPF2 being typically in the range of 10-100        kHz,    -   subtracting means SUB1 for subtracting signals A(DC) and B(DC)        and for generating said DC push-pull signal PP(DC).

The tracking system also comprises second processing means forgenerating a tracking error signal PP(AC/DC) defined by the addition ofsaid DC push-pull signal PP(DC) to a second differential signal PP′(AC).Said second processing means comprise:

-   -   a first high-pass filter HPF1 for generating a first        high-frequency signal from signal A,    -   a second high-pass filter HPF2 for generating a second        high-frequency signal from signal B,    -   amplitude detection means AD1 for detecting the amplitude A(AC)        of said first high-frequency signal,    -   amplitude detection means AD2 for detecting the amplitude B(AC)        of said second high-frequency signal,    -   subtracting means SUB2 for subtracting signals A(AC) and B(AC)        and for generating said AC push-pull signal PP(AC),    -   amplification means AMP for amplifying the AC push-pull signal        PP(AC) by a gain factor K, and for generating said second        differential signal PP′(AC),    -   adding means ADD for adding the DC push-pull signal PP(DC) to        said second differential signal PP′(AC) and for generating said        tracking error signal PP(AC/DC).        The tracking error signal PP(AC/DC) can thus be expressed by:        PP(AC/DC)=PP(DC)+PP′(AC)  Eq.1        PP(AC/DC)=PP(DC)+K*PP(AC)  Eq.2        PP(AC/DC)=A(DC)−B(DC)+K*[A(AC)−B(AC)]  Eq.3

The cut-off frequency of high-pass filters HPF1 and HPF2 is preferablysituated between the wobble frequency of the optical disc (if a wobbleis present) and the frequency corresponding to a multiple D of the clockfrequency, the wobble frequency being derived from the spiral pre-groovestructure of the optical disc, the clock frequency being a parameterdefined by the optical reading/writing standards such as the DVDstandard. The digital data written on optical discs being represented bymarks of varying length, the length of the marks and of the spacesbetween the marks are converted into an integral number of times theclock cycle length. Dealing with a run-length-limited (RLL)-coding ofthe data, this integral number can take the values d+1, d+2, . . . ,k+1. For example, if data are coded by the EFM-coding method used inCompact Disc, d=2 and k=10. If data are coded by the 17PP coding method(17PP standing for “17 Parity Preserve” described in the new Blu-rayDisc standard), d=1 and k=7.

The frequency content of the data corresponds to periods of typically2(k+1) times the clock cycle length to 2(d+1) times the clock cyclelength. The multiple D is then chosen such that D=2*(d+1), so that thecut-off frequency is below the main frequency components of the data.

For detecting the amplitude, amplitude detection means AP1 and AP2 maycomprise a rectifier 401 followed by a low-pass filter 402, as shown inFIG. 4.

In a preferred embodiment, the tracking system comprises normalizationmeans (not shown In FIG. 2) for normalizing the tracking error signalPP(AC/DC). Three normalizations can be envisaged for normalizing thetracking error signal PP(AC/DC): ps Normalization 1: $\begin{matrix}{{{PP}\left( {{AC}/{DC}} \right)} = \frac{{A({DC})} - {B({DC})} + {K^{*}\left( {{A({AC})} - {B({AC})}} \right)}}{{A({AC})} + {B({AC})}}} & {{Eq}.\quad 4}\end{matrix}$Normalization 2: $\begin{matrix}{{{PP}\left( {{AC}/{DC}} \right)} = {\frac{{A({DC})} - {B({DC})}}{{A({DC})} + {B({DC})}} + {K^{*}\frac{{A({AC})} - {B({AC})}}{{A({AC})} + {B({AC})}}}}} & {{Eq}.\quad 5}\end{matrix}$Normalization 3: $\begin{matrix}{{{PP}\left( {{AC}/{DC}} \right)} = \frac{{A({DC})} - {B({DC})} + {K^{*}\left( {{A({AC})} - {B({AC})}} \right)}}{{A({DC})} + {B({DC})} + {{K}^{*}\left( {{A({AC})} + {B({AC})}} \right)}}} & {{Eq}.\quad 6}\end{matrix}$

The normalisation of the tracking error signal is performed foraccommodating variations in the overall intensity of the light source orthe average disc reflectivity.

In a preferred embodiment, the tracking system comprises adjusting meansADJ for adjusting the value of the gain factor K.

Let push-pull ratio R be the ratio between the DC-push-pull amplitude inan area with only unwritten tracks and the DC-push-pull amplitude in anarea with only written tracks.

Let push-pull variation V be the difference in DC push-pull amplitudebetween two adjacent tracks of which one is written and the otherunwritten.

Theoretically, the push-pull variation V increases linearly with thepush-pull ratio R. Moreover, if the push-pull variation V=0, thepush-pull ratio R=1. In this ideal case the method according to theinvention eliminates both the deviation of the push-pull ratio R fromR=1 and the deviation of the push-pull variation V from V=0.

In practice, this correlation between push-pull ratio R and push-pullvariation V does exist, but an offset is present depending on, forexample, the exact chemical composition of the phase-change layer of arewritable optical disc. It means that there is a push-pull variationV≠0 if the push-pull ratio R=1. The gain factor K is then set to a valuethat optimizes the trade-off between push-pull ratio R and push-pullvariation V. The gain factor K is set to a value for which |R−1| and |V|are as small as possible.

The optimum trade-off between push-pull ratio R and push-pull variationV is found in calculating the minimum of a merit function depending on|R−1| and |V|. For example, a first merit function F1 and a second meritfunction F2 are defined by:F1(R, V)=|R−1|+|V|  Eq.7F2(R, V)={square root}{square root over ((R−1)² +V ²)}  Eq.8

FIG. 3A shows a radial tracking error signal PP(AC/DC) generated by atracking system according to the invention (bold line), and a radialtracking error signal PP(DC) generated by a tracking system according tothe prior art method (dotted line), as a function of the radial positionof an optical disc.

The zero-crossings of these radial tracking error signals are used toidentify the centers of tracks shown in FIG. 3B. The tracks are composedof alternating written tracks (grey color) and unwritten tracks (whitecolor).

Unlike the PP(DC) signal that presents large variations and asymmetry,the PP(AC/DC) is very close to an ideal sine wave. As a consequence, thelocal offset and the slope at the zero-crossings are the sameirrespective of whether the zero-crossings correspond to written orunwritten tracks.

The invention has been described with reference to the use of aphotodetector having two detection areas A1 and A2. Of course, each areaA1 and A2 may be composed of a plurality of elementary detection areas.In this case, considering the area A1 (or A2), the output signal A (andB) is generated by summation of the elementary output signals generatedby the plurality of elementary detection areas.

The invention is not limited to the use of the merit functions F1 and F2defined above, and other merit functions could be defined for finding anoptimum trade-off between push-pull ratio R and push-pull variation V.

This tracking system according to the invention may be implemented bydigital processing means (e.g. digital low-pass and high-pass filters,signal processors, memory devices), or alternatively by analogprocessing means (e.g. analog low-pass and high-pass filters).

The tracking system according to the invention is preferably implementedin an apparatus for reading and/or writing optical discs of the R type(Recordable: write-once, read-many) and of the RW type (ReWritable:write-many, read-many).

1. Tracking system for guiding an optical beam on tracks on aninformation carrier, said tracking system comprising a photodetector(A1, A2) for detecting optical beams derived from said optical beam,said photodetector generating a first output signal (A) and a secondoutput signal (B), said tracking system comprising first processingmeans for generating a first differential signal (PP(DC)) correspondingto the low-frequency part of a difference between said first and secondoutput signals, characterized in that said tracking system comprisessecond processing means for generating a tracking error signal(PP(AC/DC)) defined by the addition of said first differential signal(PP(DC)) to a second differential signal (PP′(AC)), said seconddifferential signal corresponding to a fraction of the difference inamplitude of the high-frequency components of said first and secondoutput signals.
 2. Tracking system as claimed in claim 1 comprisingadjusting means for adjusting the value of said fraction to a valuewhich minimizes a merit function (F1, F2), said merit function being afunction of a first parameter (R) and a second parameter (V), said firstparameter (R) corresponding to the ratio between the amplitude of saidfirst differential signal (PP(DC)) in an area with only unwritten tracksand the amplitude of said first differential signal (PP(DC)) in an areawith only written tracks, said second parameter (V) corresponding to thedifference in amplitude of said first differential signal (PP(DC)between two adjacent tracks of which one is written and the other isunwritten.
 3. A method of tracking for guiding an optical beam on trackson an optical disc, said method comprising a photodetection step fordetecting optical beams derived from said optical beam, saidphotodetection step generating a first output signal (A) and a secondoutput signal (B), said method comprising a first processing step forgenerating a first differential signal (PP(DC)) corresponding to thelow-frequency part of a difference between said first and second outputsignals, characterized in that said method comprises a second processingstep for generating a tracking error signal (PP(AC/DC)) defined by theaddition of said first differential signal (PP(DC)) to a seconddifferential signal (PP′(AC)), said second differential signalcorresponding to a fraction of the difference in amplitude of thehigh-frequency components of said first and second output signals.
 4. Amethod of tracking as claimed in claim 3 comprising an adjusting stepfor adjusting the value of said fraction to a value which minimizes amerit function (F1, F2), said merit function being a function of a firstparameter (R) and a second parameter (V), said first parameter (R)corresponding to the ratio between the amplitude of said firstdifferential signal (PP(DC)) in an area with only unwritten tracks andthe amplitude of said first differential signal (PP(DC)) in an area withonly written tracks, said second parameter (V) corresponding to thedifference in amplitude of said first differential signal (PP(DC))between two adjacent tracks of which one is written and the other isunwritten.
 5. Apparatus for reading data on an optical disc, saidapparatus comprising a tracking system as claimed in claim
 1. 6.Apparatus for writing data on an optical disc, said apparatus comprisinga tracking system as claimed in claim 1.